The present invention relates to a method of electrochemical machining.
Electrochemical machining enables the formation of complicated shapes. In a conventional electrochemical machining process, a shaped electrode and a workpiece are positioned in an electrolyte bath. The electrolyte is continuously replenished and a potential is applied across a gap between the electrode and the workpiece. The electrode is then fed towards the workpiece. Material is electrochemically dissolved from the surface of the workpiece in accordance with Faraday""s Law, so that the surface of the workpiece erodes in proportion to the local current density. Those parts of the workpiece which are closest to the electrode erode fastest, so that as the electrode is fed towards the workpiece it eventually adopts a surface morphology which is determined by the shape of the electrode (as well as other factors such as the feed rate, potential, and total current density).
On the other hand, wet chemical etching is sometimes used when it is desired to uniformly erode surface material, e.g. for thinning the wall thicknesses of honeycomb structures. Using this process, material removal is uniform over all the exposed surfaces of the honeycomb.
Honeycomb structures are used in many engineering applications which require a combination of lightness and strength, e.g. as a core material for structural panels of vehicles. Typically such structures have cells with hexagonal cross-sections arranged in a hexagonal array. However, other cross-sections, such as circular, square or triangular, are also possible; and other arrangements of cells can also be used, e.g. a square arrangement.
Aluminium honeycomb panels are used as crash energy absorbers in vehicle impact testing rigs. When a vehicle impacts into such a panel, the kinetic energy of the vehicle is absorbed by controlled crumpling and deformation of the honeycomb cells. The energy is absorbed in an almost entirely plastic manner with little elastic recoil, so that the amount of deformation is an accurate measure of the energy of the impact.
The deformation resistance is of course dependent on the wall thickness of the honeycomb, and the deformation resistance of a honeycomb panel can be reduced by wet etching. This can be done, for example, where it is required to vary the energy-absorbing properties of such a panel through the thickness of the panel. Two or more honeycombs with different wall thicknesses are bonded together to form a multi-layer panel, each layer having cell walls of a different thickness.
Significantly, however, the interfaces between adjacent layers form discontinuities which disturb the otherwise controlled deformation of the panel on impact. This reduces the accuracy with which the amount of energy absorption can be measured.
The present invention is concerned with the use of electrochemical machining or electrochemical etching to give a cell of a honeycomb structure a cross-section that varies in size in the direction perpendicular to that cross-section, i.e. along the axis of the cell.
The present invention provides an electrochemical machining process for giving a cell of a honeycomb structure a 3-dimensional profile form having a varying cross-section perpendicular to the axis of the cell, in which process an electrode is maintained in a constant position relative to said axis and the surface of the cell is exposed differentially to the action of the electrode in the axial direction to vary the amount of material removal and so vary said cross-section.
An advantage of this process over conventional electrochemical machining is that differential material erosion can be effected even without feeding the electrode towards the eroding surface, although the amount of erosion will decrease as the gap between the electrode and the eroding surface increases.
In one form of the invention, to give the required variation the electrode may be held stationary and screening means can selectively screen the surface of the cell from the action of the electrode.
In another form of the invention, the electrode can be moved relative to the workpiece along said axis to give the required variation.
In a further form of the invention, to give the required variation the electrode can be shaped to provide a gap between the electrode and the surface of the cell, the gap having a varying thickness perpendicular to said axis.
The present invention finds particular application for the production of honeycomb panels (e.g. made of aluminium or aluminium alloy) having honeycomb cell wall thicknesses which vary in the axial direction of the cells.
To machine a honeycomb cell or cavity, preferably the electrode is elongate and oriented parallel to the axis of the cavity, and has a cross-sectional shape which corresponds to the cross-sectional shape of the cavity. The erosion rate can then be controlled to occur at the same rate around the periphery of the cavity, even in cylindrical cavities of non-circular cross-section such as hexagonal honeycomb cells.
The cell wall thicknesses of the honeycomb can be varied for all or some of the cells so that the energy-absorbing properties of the honeycomb vary in a progressive and determinate manner through the thickness of honeycomb. An advantage over conventional honeycomb panels formed from layers of honeycombs is that it is possible to avoid forming discontinuities which affect the energy absorbing properties of the wall.
A preferred method of the present invention is particularly suitable for the machining of the walls of honeycomb cells where the cells are irregularly shaped and spaced. Irregularly shaped and spaced cells result from nature of the adhesive bonding process used to make e.g. aluminium honeycombs. Irregularly shaped and spaced cells make it more difficult to (a) centre the electrode in any particular cell, and (b) ensure that uniform erosion takes place around the periphery of the cell.
Therefore, the preferred method includes inserting one or more wedging fingers in respective neighbouring cells, the wedging fingers being spaced from the electrode and engaging with the walls of their respective cells so that the honeycomb structure is locally forced to become regular and the electrode is maintained at the axis of its cell.
Preferably the support fingers are tapered to facilitate their insertion into their respective cells.
The present invention also encompasses an apparatus for electrochemical machining a honeycomb structure, the apparatus including at least one elongate electrode and one or more wedging fingers spaced from the electrode. When the honeycomb has hexagonal cells, preferably the or each electrode is surrounded by six wedging fingers which form the vertices of a regular hexagon centred on the electrode.
Preferably the apparatus further includes means for applying an electrical potential between the electrode and the honeycomb structure, and means for supplying a flow of electrolyte to the gap between the electrode and the surface of a honeycomb cell. Additionally, the apparatus has (a) means for providing along the axis of the cell relative movement between the electrode and the cell and/or (b) means for selectively screening the surface of the cell from the electrode. The means for selectively screening the surface of the cell may include an insulating sleeve, for occupying a corresponding gap between the electrode and the surface of the cell, and means for providing along said axis relative movement of the sleeve and the honeycomb structure.
The present invention further encompasses a honeycomb structure which has been electrochemically machined by the process of the invention so that the walls of some or all of the cells vary in thickness in the axial direction of the cells.
The present invention will now be described in relation to specific embodiments and with reference to the accompanying drawings, in which: